Re: [Phys-L] irreversible quasistatic

[John Denker <jsd@av8n.com>]

On 07/29/2016 11:13 AM, Carl Mungan wrote:
> I’m assuming you meant curve B is supposed to deviate somehow. What
> is it missing?


Consider the following system:  A heat engine drives a shaft which
is connected to a mechanical brake.  The brake is inside the system
boundary.  The subsystem consisting of the heat engine is reversible,
but the overall system is grossly irreversible.

The PV "indicator diagram" for the heat engine is the same as it
would be without the brake.  It's just not the whole story.

Area on the indicator diagram has dimensions of energy, but there
is more to physics than dimensional analysis.  For a reversible
system, the PdV work must be delivered across the system boundary,
because there is nowhere else for it to go.  As soon as you add
in the irreversible brake, there are additional terms in the
equations.

Using a Joule-Thompson plug makes the braking action internal
to the gas, but it's the same idea.